Japanese Patent Laid-open Publication No. 1999-248671, describes a gas sensor that may be employed in an oxygen measuring device of an air-fuel ratio control system. The gas sensor measures an oxygen content in exhaust gasses of an internal combustion engine of an automotive vehicle and so on.
As shown in FIG. 20, a gas sensor 9 holds therein a sensor element 910 that detects a concentration of a particular gas in a measurement gas. A housing 911 holds therein a sensor element 910 via an insulation porcelain 913. An air cover 92 is installed on the base portion of the housing 911.
An elastic insulating member 93 is installed in the base portion of the air cover 92. The elastic insulating member 93 receives lead wires 912 that make an electric contact at one end with the sensor element 910.
An outer cover 94 is installed on the base portion of the air cover 92. The outer cover 94 is calked in a radially inner direction at three axially spaced calking portions 940.
Furthermore, an air permeability filter 95 having a water-proof property is held between the air cover 92 and the outer cover 94 and crimped to be retained by two of the calking portions 940. Air as a reference gas is introduced from a through hole 941 in the outer cover 94 to an air vent 921 of the air cover 92 via the air permeability filter 95. After that the air is introduced into gas sensor 9.
In this conventional gas sensor 9, however, since the air permeability filter 95 is retained between the air cover 92 and the outer cover 94, the air permeability filter 95 easily receives heat via the air cover 92 or the outer cover 94. Therefore, when the gas sensor 9 is used in a exhaust of the internal combustion engine any number of times, the permeability filter 95 is subject to heat deterioration and the waterproof property at the calking portions 940 that retain the permeability filter 95 may fail. Thus, there is the concern that outside water will infiltrate into the gas sensor 9 through the air vent 921.